1. Field of the Invention
This invention relates to dye-receiving sheets for thermal transfer printing which are used in combination with a transfer sheet using sublimable dyes.
2. Description of the Prior Art
Dye-receiving sheets of the type to which the present invention is directed are known, for example, in Japanese Laid-open Patent Application No. 59-133098. This patent application describes a sublimable dye-receiving layer which is made of a ultrafine oxide powder as a dye adsorber and a polymer material for dispersing the powder.
Japanese Laid-open Patent Application No. 59-215398 describes a dye-receiving sheet having a coating or impregnated layer of a thermoplastic polyester resin and a crosslinked polyester resin.
The fundamental characteristics required for an image or dye-receiving sheet for thermal transfer printing processes using sublimable dyes include good dye receptivity, anti-blocking properties against a dye-bearing layer of a dye transfer sheet, e.g. anti-fusing properties and a lowering in releasability both at the time of thermal printing, and weatherability sufficient to maintain a stable dye-receiving state. In addition, since the sheet is an article of consumption, production costs should be low.
In order to ensure good dye receptivity and weatherability, the dye-receiving layer should basically contain a large proportion of a dye-receiving resin capable of stably receiving a sublimable dye and readily dispersing the dye. Such dye-receiving resins are usually thermoplastic resins, typical of which are saturated linear polyester resins. However, if these dye-receiving resins are used singly as the dye-receiving layer, a good heat resistance and releasability cannot be obtained, or good anti-blocking properties cannot be ensured. To avoid this, usual practice is (1) to add heat-resistant fine particles and a lubricant or release agent to the dye-receiving resin so as to impart a high heat resistance and releasability, or (2) to use a crosslinking resin component as part of the dye-receiving resin and cure this component after formation of a dye-receiving layer by which the layer is imparted with a good heat resistance and releasability. The sheet described in the Japanese Laid-open Patent Application No. 59-122098 is of the former type and the sheet set forth in the Japanese Laid-open Patent Application No. 59-215398 is of the latter type.
In the above class (1), large amounts of heat-resistant fine particles or a lubricant or releasing agent are necessary for imparting antiblocking properties of a level sufficient for practical use. These ingredients are generally used in amounts not less than 50 wt% of the total solid components in the dye-receiving layer. This means a reduction in amount of the resin which actually receives dye molecules. The resultant layer becomes poorer in color fastness to light and color fastness in the dark than a case where a larger amount of the resin is used, with a poorer sensitivity to color fomation. Moreover, the fine particles used in large amounts worsen the transparency and gloss of the dye-receiving layer, thus impeding the brightness of a color-developed image.
In the second class, the problems involved in the first class can be solved, but when the crosslinking density increases and the heat resistance of the dye-receiving layer is thus improved so as to enhance the anti-blocking properties, the diffusability of dye molecules decreases with a lowering of chromophoric sensitivity. Moreover, the curing process is an additional step by which the productivity lowers with respect to the formation of the dye-receiving layer.